As one of thermistors, a PTC (Positive Temperature Coefficient) thermistor with a positive temperature coefficient of resistance has been known. The PTC thermistor will have an increase in the resistance against the increase of the temperature, thus it can be used as a self-regulated heater element, an over current protection element, an overheat detection element and the like. In the prior art, a PTC thermistor has been obtained by adding a minute amount of rear earth elements and the like to barium titanate (BaTiO3) which is the main component to form a semiconductor. Although it has a low resistance below the Curie Point, it will have a sharp increase in the resistance by several orders of magnitude above the Curie Point.
The Curie Point of BaTiO3 is usually about 120° C. However, it can be shifted to a lower temperature by replacing a part of Ba and Ti with Sr or Sn and Zr. Especially, the Curie Point is required to be high for the PTC thermistor used as a heater because the PTC thermistor with high Curie point generates high temperature heat source. However, as for the shift of the Curie Point to higher temperature, it has been realized by replacing a part of Ba with Pb at present. From the view point of the trend of decreasing the environmental load of the world, practical application of a replacement material without using Pb has been demanded.
In the following Patent Document 1, a method for producing a semiconductor ceramic composition has been disclosed. In the method, one or more of any of Nb, Ta or a rare earth element are added into a composition consisting of Ba1-2x(BiNa)xTiO3 (0<X≤0.15), in which a part of Ba is replaced with (BiNa) rather than Pb. Then, after the composition is sintered in nitrogen, it is heat-treated in an oxidation atmosphere.
In addition, in the following Patent Document 2, a method for producing a semiconductor ceramic composition has been disclosed. In the method, for a semiconductor ceramic composition in which a part of Ba in BaTiO3 is replaced with Bi and alkali metal(s) (which is at least one selected from the group consisting of Na, K and Li), as a manner applied to suppress the resistance change rate of the resistance value after electrifying the semiconductor ceramic composition for a long time, the ratio of the measured density relative to the theoretical density (herein after, referred as “relative density”) of the sintered compact of the composition is set to be 70˜90%.
All of the above mentioned Patent documents have disclosed that, a semiconductor ceramic composition without using Pb, which has a Curie Point shifted to a temperature higher than 120° C., and a small resistivity at 25° Ccan be obtained.